Mock is a paleoclimatologist: He studies weather patterns over the Earth's entire history, and uses them to forecast meteorological patterns in the future. With the information he gathers from old instrumental records, newspaper reports and personal diaries, he draws weather maps similar to the kind used by television weather people, only for weather events that happened long before radar was around to analyze Doppler effects and bow echoes. Lee's diary helped Mock grasp the extent of the turmoil: "It's hard to say that the winter storms of 1862 were unprecedented if you just pinpoint one location," he says. "But in terms of how widespread it was, nothing we have in the historical record even comes close."

In the Washington Territories, reported the Olympia Overland Press, "It raineth every day, and every night also." In Nevada, according to a letter-writer named Uriah Allen, the Carson River turned the Carson Valley into a "muddy lake," and "all the bridges upon the rivers were washed away." An assiduous California diarist named Alfred Doten reported that "the water covers all Sacramento Valley, & the cattle are dying in droves -- they stand in 2 or 3 ft of water, in huddles & starve to death -- no chance to save them." Farther south, an entire town, christened Agua Mansa for the formerly gentle waters of the Santa Ana River, completely disappeared -- a calamity documented in the diary of a regional judge named Benjamin Hayes. Hayes recalled the local priest frantically ringing the church bell to call the townspeople to higher ground. Miraculously, no one was killed.

How will modern dams, river channels and levees -- most of which are built only to handle 50-year floods -- fare when waters rush out of the mountains in "billows 50 feet high," as one diarist described it? Keith Porter, a structural engineer at the University of Colorado at Boulder, suggests that an 1862-type storm could cause 300 miles of flooding in California's Central Valley, with waterways anywhere from 12 to 60 miles wide. And while "we're not ready to release all the detail of our flooding scenario just yet," Porter says, "we imagine large areas of Orange County will flood -- in some places several feet deep." Fuel lines, aqueducts, interstate highway bridges would all be vulnerable to failure.

It's too soon in the ARkStorm process to predict the extent of the suffering an extended atmospheric river would cause in California's most vulnerable region -- the increasingly fragile California Bay-Delta, at the confluence of the state's two largest rivers. But we can begin to imagine it: In Marc Reisner's A Dangerous Place: California's Unsettling Fate, the author hypothesized what would happen if an earthquake on the Hayward Fault under Oakland caused the Delta's levees to fail. If the earthquake hit in the spring, "with the Sacramento River carrying most of the Sierra Nevada's runoff," 10,000 acres of dry land would be "filled to sea level in a couple of hours." As other levees failed in turn, seawater would push into the Delta, contaminating the source of half the state's drinking water. A catastrophic flood would have the same effect.

So what's a public policy planner to do? Can we protect our cities against the coming flood? The answer, for the most part, is no. Even though Dettinger projects that climate change may cause atmospheric rivers to hit the West Coast with increasing frequency, and despite all the promises governors have made to fix their states' infrastructure, neither federal dollars nor state budgets will be enough to shore up the walls against a once-a-century storm. "The question we're trying to answer instead," Porter says, "is: ‘What we will do to mitigate the damage in terms of coordinating emergency response?' "

The ARkStorm team has until May of 2011 to design an emergency response drill around that question. And some of the answers might turn out to be simple. Up in Sacramento, Cox talked to fire department Battalion Chief Marc Bentovoja about the possibility of the city flooding as it did in 1862. The newly elected governor, Leland Stanford, had had to travel by rowboat from San Francisco to his inauguration at the state's Capitol. "(Bentovoja's) response was, ‘I don't have enough boats,' " Cox recalls. "But the community has a lot of boats. I suggested to him that local responders might organize boats for an event. Count the boats!"

Contributing Editor Judith Lewis writes about the environment from Venice, California.

For more information, please see:

The USGS's Multihazard demonstration project, including ARkStorm.

NOAA on atmospheric rivers

Flooding from atmospheric rivers